1. FIELD OF THE INVENTION
The present invention relates to a method for producing an imide oligomer soluble in an organic solvent, which is excellent in the stability and moldability, and it provides a precursor substance which is thermally self-curable without producing a volatile substance such as water or an alcohol to form a polyimide resin having excellent heat resistance. More particularly, it relates to a method for producing an imide oligomer suitable for the production of a fiber-reinforced composite material having excellent heat resistance, which has glass fibers, aramide fibers, carbon fibers, almina fibers or silicon carbide fibers as a reinforcing material.
2. DISCUSSION OF BACKGROUND
It has already been known that a polyimide resin having excellent heat resistance can be obtained by the reaction of an aromatic diamine with pyromellitic dianhydride PMDA or 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) or with its derivative such as its acid or lower alkyl ester. Such a polyimide is usually insoluble and infusible, and its moldability is extremely poor. Therefore, the molding is conducted in the form of its precursor such as polyamide acid or polyamide ester, followed by the conversion of the molded precursor to imide. During the conversion to imide, volatile substances such as water and an alcohol are produced, and when such a process is applied to a fiber-reinforced composite material, voids are likely to be formed, such being undesirable. Further, a solution of polyamide acid or polyamide ester is usually highly viscous, and it is difficult to prepare a prepreg. Besides, the stability of such a precursor is so poor that it is practically impossible to store the prepreg for a long period of time.
U.S. Pat. No. 3,745,149 discloses a method for producing a polyimide which is applicable to a fiber-reinforced composite material. This method comprises preparing a prepreg from a solution of a three-component mixture of an aromatic tetracarboxylic acid dialkyl ester, an aromatic diamine and an unsaturated dicarboxylic acid alkyl ester, followed by molding. According to this method, the preparation of the prepreg can easily be conducted by the reduction of the viscosity of the solution, but the stability of the prepreg is not adequate, and a low temperature is required for the storage for a long period of time. Further, the molding requires a two step reaction comprising formation of a precursor and curing, whereby volatile substances such as water and an alcohol are formed, thus leading to formation of voids and poor moldability.
The present inventors have previously filed a patent application for a method of producing a solvent-soluble imide oligomer as a precursor for a polyimide resin for a composite material, having the storage stability and moldability improved (Japanese Unexamined Patent Publication No. 167569/1984). By using such an imide oligomer, the storage stability and moldability can be improved, but the heat resistance is not adequate.